Two-layer system

ABSTRACT

Radiation-curable two-layer systems containing immobilized photoinitiators are suitable for the production of coatings of constant thickness and good adhesion, and relief structures of high resolution and edge steepness.

This is a division of application Ser. No. 07/090,283 filed Aug. 28,1987, now U.S. Pat. No. 4,914,004.

BACKGROUND OF THE INVENTION

The invention relates to a two-layer system which can be cured byradiation and to the use thereof for the production of polymericcoatings.

Radiation-curable systems are materials which are sought after and arefrequently employed in the production of photopolymerized coatings forsurface finishing and in the production by photolithography ofphotopolymerized relief structures. The main field of use in thisrespect is the production of paint coatings on a very wide variety ofsubstrates. In addition, such materials are used, in the production ofmicroelectronic and optoelectronic components and switch gear, as aphotoresist in the production of switching structures or in the latteras protective or passivating layers having special resistance to heatand chemicals, as insulating layers or as dielectrics or, inliquid-crystal display cells, as orientation layers. Furtherapplications are use as an etch resist and plating resist and as asolder resist in the production of printed circuits and printing platesand other reprographic materials. Finally, systems of this type are alsosuitable for the preparation of anisotropic polymer coatings for use inthe field of non-linear optics.

The multifarious methods of preparing radiationcurable systems can beinferred, for example, from W. S. De Forest: Photoresist Materials andProcesses, McGraw Hill, N.Y., 1975, and also German PatentSpecifications 2,380,830, 2,437,348 and 2,722,264 and European PatentSpecification 3,002.

The processes and methods hitherto known exhibit a number ofdisadvantages. Thus their sensitivity is frequently unsatisfactory, as aresult of which exposure times of several minutes are required, usingthe customary radiation sources, for adequate photo-crosslinking and thelayer hardness caused thereby and for ensuring the resolution and edgesteepness in relief structures which are necessary in microelectronics.Secondly, inadequate resolving power and unsatisfactory edge steepness,caused, for example, by under-irradiation of the copying layer or byreflections at the substrate, are a source of criticism of the knownphotopolymerizable systems.

A further problem in coatings of all kinds is the adhesion of the filmto the substrate. Detachment of the coating from the substrate occursfrequently, particularly in the case of substrates of an irregular shapeor materials subjected to considerable bending stresses.

Finally, there is often a need for coatings which have a homogeneousthickness within a wide area, which in most cases can only be achievedwith difficulty by means of conventional coating techniques.

SUMMARY OF THE INVENTION

It is an object of this invention to provide a radiation-curable systemwhich makes it possible to prepare coatings of constant thickness andgood adhesion and, in the case of the production of photolithographicstructures, permits relief structures having a higher resolution andedge steepness.

Upon further study of the specification and appended claims, furtherobjects and advantages of this invention will become apparent to thoseskilled in the art.

These objects are achieved by means of a radiation-curable two-layersystem comprising a layer which has been applied to a substrate andcontains initiators and a superposed layer containing polymerizablemonomers or oligomers, characterized in that the initiator isimmobilized on a macrostructure.

Two-layer systems which can be cured by radiation are known. Thus U.S.Pat. No. 4,551,418 teaches the coating of a substrate by means of acationic photoinitiator, the exposure of the latter through a copyinglayer with the production of cationic, initiating molecular fragmentsand bringing the initiator layer into contact with a monomer which isamenable to cationic polymerization and polymerizes at the exposedpoints. A relief structure is obtained after the non-polymerizedportions of the layer have been removed by washing. However, coatingsproduced by this process suffer from the same disadvantages indicatedabove. In particular, the adhesion to the substrate and edge steepnessof the polymerized system are inadequate, and the choice of monomers isrestricted to the relatively small number of compounds accessible tocationic polymerization.

It has now been found, surprisingly, that a two-layer system containingan initiator immobilized on a macrostructure can be polymerized byradiation curing to give coatings of constant thickness and goodadhesion and relief structures of high resolution and edge steepness.

The nature of the initiators used is not restricted. It is possible toemploy initiators which decompose under the influence of radiationeither by a free radical or ionic mechanism and which are suitable forthe particular monomers or oligomers.

Immobilization of the photoinitiator can be achieved, for example, bychemical bonding to or on surfaces, in inclusion compounds of theclathrate type, or by embedding in rigid, macro-reticular structures orin polymers by melting or dissolution processes.

For example, a substrate coating in which the mobility of the initiatoris considerably restricted is obtained by dissolving a photoinitiator ina polymeric material, applying the solution to a surface and removingthe solvent by evaporation, or by applying a melted polymeric materialcontaining a photoinitiator and allowing this material to solidify.

It is preferable to immobilize the initiator by means of a covalentbond. In a preferred embodiment of the present invention, the initiatoris attached by a covalent link to a polymer matrix.

Initiators suitable for bonding to the polymer matrix by a covalent bondcorrespond, for example, to the formula I

    Ar-CO-R                                                    I

wherein ##STR1## R is Ar or CR² R³ R⁴, R¹ is H, halogen, C₁ -C₁₂ -alkyl,C₁ -C₁₂ -alkoxy, C₁ -C₁₂ -alkylthio or A,

R² and R³ independently of one another are each H, C₁ -C₆ -alkyl orphenyl, one of these radicals can also be C₁ -C₆ -alkylene-O-A, or R²and R³ together are C₂ -C₆ -alkylene or C₁ -C₆ -oxaalkylene,

R⁴ is OR⁵, N(R⁵)₂, SR⁵, or A,

R⁵ is H, C₁ -C₆ -alkyl or C₁ -C₆ -alkanoyl,

A is ##STR2## X is O, NH or (C₁ -C₆ -alkyl)-N, Y is O, N or S,

Z is in each case independently of one another H or CH₃,

m is 0 to 4 and

n is 2 to 10⁶,

subject to the proviso that the compounds of the formula I contain atleast one radical of the formula A.

Compounds of this type are described, for example, in EP-A-O,161,463 andDE-A-3,534,645.

Initiators which are attached by a covalent link to a substrate surfaceare also preferred within the scope of the present invention. In thiscontext, a substrate is to be understood as meaning any coatablesurface, such as, for example, paper, cardoard, wood, glass, metal orpolymeric materials. A bond is created between the reactive groupslocated on the surface of these materials and the initiator, which isalso equipped with a reactive group. Particularly suitable reactivegroups are appropriately substituted silanes of the formula I whereinAr, R, R¹, R², R³, R⁴, R⁵, X, Y, Z, m and n have the meaning indicatedabove and A is here a radical

    --W-Si-R.sub.a.sup.6 R.sub.b.sup.7 R.sub.c.sup.8

wherein

R⁶, R⁷, and R⁸ are each independently H, F, Cl, Br, I, CN, NC, OCN, NCO,SCN, NCS or N₃, C₁ -C₁₅ -alkoxy or C₁ -C₁₅ -alkanoyl,

W is a direct bond or is C₁ -C₂₀ -alkylene wherein one or twonon-adjacent CH₂ groups can also be replaced by O, N or S, and a, b andc are 0, 1, 2 or 3,

subject to the proviso that a+b+c=3 and at least one of the substituentsR⁶, R⁷ and R⁸ is other than hydrogen.

Examples of initiators containing silyl groups of this type aredisclosed in EP-B 3,002 or can be prepared by methods such as can beseen, for example, in DE-A 3,521,201 or DE-A 3,601,742.

Suitable polymerizable compounds are mono-ethylenically orpoly-ethylenically unsaturated monomers, oligomers or prepolymers ormixtures thereof which are capable of polymerization which can beinitiated either by means of free radicals or by ionic compounds. Thestate of the art offers a large selection of suitable compounds for thispurpose.

Examples of suitable ethylenically unsaturated compounds are ethylene,propylene, butene, isobutylene, butadiene, isoprene, vinyl chloride,vinylidene chloride, vinylidene cyanide, acrylic acid, methacrylic acid,acrylonitrile, methacrylonitrile, acrylamide, methacrylamide, methyl,ethyl, n-butyl, tert.-butyl, cyclohexyl, 2-ethylhexyl, benzyl,phenoxyethyl, hydroxyethyl, hydroxypropyl, lower alkoxyethyl ortetrahydrofuryl acrylate or methacrylate, vinyl acetate, propionate,acrylate or succinate, N-vinylpyrrolidone, N-vinylcarbazole, styrene,divinylbenzene, substituted styrenes and mixtures of such unsaturatedcompounds. It is also possible to employ polyunsaturated compounds, suchas, for example, ethylene diacrylate, 1,6-hexanediol diacrylate andpropoxylated bisphenol A diacrylate and dimethacrylate in the processesaccording to the invention. Ethylenically unsaturated compounds can bepolymerized by a free radical or ionic mechanism.

Examples of monomers suitable for ionic polymerization are vinyl ethers,cyclic esters and ethers, olefins or further compounds such as thoselisted, for example, in R. W. Lenz: Organic Chemistry of Synthetic HighPolymers, Interscience Publishers, New York, 1967.

The term "polymerization" should be understood in the broadest sense. Itincludes, for example, the further polymerization or cross-linking ofpolymeric materials, for example prepolymers, the homopolymerization,copolymerization and terpolymerization of simple monomers and also thecombination of the types of reaction mentioned.

The free radical or ionic fragments required to initiate polymerizationcan be formed in various ways, for example by irradiating the monomerswith UV light, X-rays or radioactive sources of radiation.

Sunlight or artificial radiation emitters can be used as sources ofradiation. Examples of advantageous emitters are high-pressure,medium-pressure or low-pressure mercury vapor lamps and xenon andtungsten lamps; it is also possible to employ laser light sources andcathode ray tubes.

The production of the two-layer system according to the invention iseffected in a simple manner by applying an initiator or mixture ofinitiators in a dissolved or liquid form to a substrate, for example bycoating, printing, dipping or spin-coating to give a macro-structureprovided with an initiator attached by covalent bonds and/or a layer ofinitiator molecules anchored by covalent bonds to the surface of thesubstrate.

The desired monomer, oligomer or mixtures thereof, if appropriate in thepresence of additives, is applied to this initiator layer by means ofthe techniques indicated above.

Additives of this type can, for example, be reaction accelerators.Examples of reaction accelerators which can be added are organic amines,phosphines, alcohols and/or thiols containing in every case at least oneCH group in an α-position relative to the heteroatom. Examples ofsuitable accelerators are primary, secondary and tertiary aliphatic,aromatic, araliphatic or heterocyclic amines, such as are described, forexample, in U.S. Pat. No. 3,759,807. Examples of amines of this type arebutylamine, dibutylamine, tributylamine, cyclohexylamine,benzyldimethylamine, dicyclohexylamine, triethanolamine,N-methyldiethanolamine, phenyldiethanolamine, piperidine, piperazine,morpholine, pyridine, quinoline, ethyl p-dimethylaminobenzoate, butylp-dimethylaminobenzoate, 4,4'-bis-dimethylaminobenzophenone (Michler'sketone) or 4,4'-bis diethylaminobenzophenone. Particularly preferredamines are tertiary amines, such as, for example, trimethylamine,triisopropylamine, tributylamine, octyldimethylamine,dodecyldimethylamine, triethanolamine, N-methyldiethanolamine,N-butyldiethanolamine, tris-(hydroxypropyl)-amine and alkyldimethylaminobenzoates.

Further examples of suitable reaction accelerators aretrialkylphosphines, secondary alcohols and thiols.

It is also possible to add small quantities of light stabilizers, suchas, for example, benzophenone derivatives, benzotriazole derivatives,tetraalkylpiperidines or phenyl salicylates.

Depending on the end use, suitable additives for the monomers to bepolymerized are organic additives, such as thixotropic agents, levellingagents, binders, lubricants, delustering agents, plasticizers, wettingagents, silicones for improving the quality of the surface, andanti-floating agents or small amounts of solvents.

The present invention also relates to a process for the production ofpolymeric coatings from the two-layer systems according to theinvention. This is effected by irradiating the monomers or oligomerspresent on the initiator layer with one of the above-mentioned lightsources, which affords a well-adhering polymer layer in which thethickness distribution is virtually homogeneous and is substantiallyindependent of the thickness of the layer of monomer or oligomeroriginally applied. After irradiation, non-cured constituents presentabove the polymer layer are removed by washing with a suitable solvent,as described, for example, by W. S. De Forest, ibid.

In a particular embodiment, the substrate which has been provided withthe layer of initiator is merely dipped into a medium containing themonomers or oligomers, and the irradiation is then carried out in theimmersed state. In this case the photopolymerization is initiated onlyin the boundary region between the photoinitiator layer and the medium,not in the further medium surrounding the substrate, so that a polymerstructure in the form of a homogeneous, well-adhering coating is thusformed only on the substrate. One advantage of this process variant isthat it is possible to coat even 3-dimensional objects on all sides in asimple manner or to employ low-viscosity monomer materials orformulations such as, for example, solutions of acrylic acid, whichcannot be applied without difficulty in the form of a stable layer ofuniform thickness, particularly on non-planar surfaces or substrateshaving a 3-dimensional shape.

The present invention also relates to a process for the production ofrelief structures by irradiating the two-layer systems according to theinvention through a copying layer or mask. The relief structuresobtained after removing the areas not exposed to irradiation by washingand hence not polymerized, are distinguished by a high degree ofresolution and great edge steepness.

Finally, the invention also relates to a process for the production, byphotolithography, of waveguide structures for integrated optics. Thus itis possible to produce structures having refractive indices differingfrom those of the adjoining material by exposing a two-layer systemaccording to the invention to light, if appropriate through a copyinglayer. Structures of this type are suitable for use as waveguides andoptical switches, for example after electrodes have been attached. Theuse of substrates, layers containing initiator or monomers havingnonlinear optical properties prove advantageous in this connection.

The properties of the polymer materials obtained can be modified in amanner which is in itself known by copolymerization or mixing withfurther components, by varying the molecular weights, by adding a verywide variety of inorganic or organic additives and metals and by manyfurther treatments with which polymer experts are familiar.

The two-layer systems according to the invention can be employed withadvantage for coating, for example, paper, glass, plastics and metalswith radiation-curable paint formulations.

By this means it is possible to produce, by photolithographic processes,relief structures which are used as printing plates, photoresists andelectronic control elements.

The field of integrated optics opens up further possible uses. Thecompositions according to the invention are suitable, in particular, forthe production of waveguides, optical switch gear and optical datastorage systems.

Without further elaboration, it is believed that one skilled in the artcan, using the preceding description, utilize the present invention toits fullest extent. The following preferred specific embodiments are,therefore, to be construed as merely illustrative, and not limitative ofthe remainder of the disclosure in any way whatsoever.

In the foregoing and in the following examples, all temperatures are setforth uncorrected in degrees Celsius and unless otherwise indicated, allparts and percentages are by weight.

The entire text of all applications, patents and publications, citedabove and below are hereby incorporated by reference.

EXAMPLES Example 1 Preparation of poly-[4-(2-acryloyloxyethoxy)-phenyl]2-hydroxy-2-propyl ketone

30 g of [4-(2-acryloyloxyethoxy)-phenyl] 2-hydroxy-2-propyl ketone, 200ml of tetrahydrofuran and 0.30 g of dibenzoyl peroxide are combined atroom temperature and the mixture is heated under reflux for 15-20 hours.The solvent is then removed on a rotary evaporator and 0.5 g of activecharcoal is added to the residue, which is stirred and filtered, and 300ml of n-hexane are then added to the filtrate. The supernatant solutionis then decanted off from the precipitated product, and thisreprecipitation process is repeated twice. After being dried at roomtemperature, the product is an amorphous, solid mass, which is readilysoluble in the customary organic solvents (acetone, methylene dichlorideor ethyl acetate); average molecular weight: approx. 4,000 (determinedby gel permeation chromatography); glass transition temperature Tg:45°-50°.

Example 2 Exposure of poly-[4-(2-acryloyloxyethoxy)-phenyl]2-hydroxy-2-propyl ketone and trimethylolpropane triacrylate to light

A solution of poly-[4-(2-acryloyloxyethoxy)-phenyl] 2-hydroxy-2-propylketone in acetone is applied to a glass surface in such a way that athin film remains after the solvent has evaporated off.Trimethylolpropane triacrylate is applied to this film. This coating iscovered at certain points and, after approx. 30 seconds, is thenirradiated with a medium-pressure Hg lamp. The coating is then washedseveral times with acetone, in the course of which it dissolves at thecovered areas and adheres to the glass surface in sharp outlines as ahard, insoluble layer at the exposed points.

Example 3 Preparation of poly-[4-(2-methacryloyloxyethoxy)-phenyl]2-hydroxy-2-propyl ketone

7.0 g of [4-(2-methacryloyloxyethoxy)-phenyl] 2-hydroxy-2-propyl ketone,100 ml of ethyl acetate and 0.15 g of dibenzoyl peroxide are combined atroom temperature, and the mixture is boiled under reflux. Furtherportions of 0.15 g of dibenzoyl peroxide are added after 2 hours andafter 6 hours, and the solution is boiled under reflux for a total of15-20 hours. The solvent is then removed on a rotary evaporator, and theresidue is dissolved in 100 ml of acetone by stirring, and the solutionis filtered. Concentrating the filtrate and drying the residue at roomtemperature gives an amorphous solid; average molecular weight: approx.12,000; Tg: approx. 55°.

Example 4 Exposure of poly-[4-(2-methacryloyloxyethoxy)-phenyl]2-hydroxy-2-propyl ketone and trimethylolpropane triacrylate to light.

A 10% solution of poly-[4-(2-methacryloyloxyethoxy)-phenyl]2-hydroxy-2-propyl ketone in acetone is spin-coated onto an aluminiumsheet at 4,500 r.p.m. for 30 seconds. The substrate coated with thepolymeric initiator is then dried at 90° for 15 minutes.Trimethylolpropane triacrylate is then spin-coated at 2,000 r.p.m. for 5seconds.

For exposure, the aluminium sheets provided with the two-layer systemare passed under two medium-pressure mercury lamps (radiation output2×80 watts/cm) on a conveyor belt at a speed of 2.5 m/minute and arethus cure.

This gives a well-adhering coating which shows no tendency at all tofracture or flake off even after the substrate has been bent severaltimes. The film thickness values determined by means of a thicknessmeasuring instrument are 0.635±0.015 μm over the whole area of thecoating.

Example 5 Production of a waveguide structure

A vinylidene polymer doped with methyl N-(2,4-dinitrophenyl)-alaninate(MAP) as specified in EP-A 0,186,999 is coated with a 10% solution inethanol of α-(3-triethoxysilylpropyl)-benzoin (DE-A 3,624,898). Afterthe solvent has been removed by evaporation at 40°, a layer oftrimethylolpropane triacrylate 5 μm thick is applied to the initiatorlayer by means of a wire-wound draw bar. The system is irradiatedthrough a copying layer for 5 minutes by means of a mercury vapor lamp.After removing uncured monomers by washing with acetone, a waveguidestructure having varying refractive indices is obtained.

Example 6 Dip coating

A piece of aluminium foil is coated with an approx. 20% solution inacetone of the polymeric photoinitiator from Example 1, and is thendried at 100° C. for 10 minutes. The coated foil is then placed in 50%aqueous acrylic acid solution and is irradiated through the solution for1 minute by means of a high-pressure mercury lamp. After being removedfrom the solution and washed with water, the foil has a well-adheringfilm of polyacrylic acid which can be swollen in water. No observablepolymerization has taken place in the monomer solution.

Example 7 (comparison)

An aluminum foil is coated with an acetone solution of the non-polymericphotoinitiator phenyl 2-hydroxy-2-propyl ketone and is then treatedfurther analagously to Example 6. On being irradiated, virtually thewhole liquid phase gels as the result of polymerization. A usablecoating is not obtained.

Example 8

An aluminum foil which has been pretreated as in Example 6 is immersedin 2-hydroxyethyl acrylate and irradiated in an analogous manner. Awell-adhering, gelatinous coating is obtained. No change takes place inthe liquid monomer phase.

Example 9

A clay crock is immersed in an approx. 20% acetone solution of thepolymeric photoinitiator from Example 1 and is then dried at 100° C. for10 minutes. It is then hung in a 50% aqueous solution of acrylic acidand irradiated through the solution from all sides. After being takenout of the monomer solution, which has otherwise remained unchanged, andafter being washed with water, the crock is encased on all sides by auniform layer of polymer.

Example 10

About 1 ml of a 20% acetone solution of the polymeric photoinitiatorfrom Example 1 was applied to a sheet of glass (5 cm×5 cm) and was thenapplied as a homogeneous layer by the spin-coating process (1 second at2,000 r.p.m.). The coated sheet of glass was heated at 90° C. for 15minutes in a drying cabinet in order to remove the solvent. Thethickness of the layer applied was then found to be 1.9-2.1 μm by meansof a film thickness measuring instrument ("Alpha-step 200", TencorInstruments).

The sheet of glass was then placed in a quartz dip pipe used as areaction vessel and was covered with a solution consisting of 97 partsby weight of hexane, 5 parts by weight of 2-propanol and 3 parts byweight of trimethylolpropane triacrylate. A reflux condensor was fittedon top of the reaction vessel.

A UV lamp (type TQ 180, Heraeus Original Hanau) was set up parallel tothe dip pipe at a distance of 7 cm so that the sheet of glass coatedwith the polymeric photoinitiator, located in the dip pipe, could beirradiated with UV light in an optimum manner. After being irradiatedfor 20 minutes, the irradiated sheet of glass was taken out of thereaction vessel, rinsed with acetone and then dried at 90° C. for 10minutes. The thickness of the film was then measured. It was 2.5-2.6 μm.The film has, accordingly, become about 0.5 μm thicker during theirradiation with UV light as the result of polymerization of thetrimethylolpropane triacrylate from the solution. In a subsequent test,carried out in a similar manner, the exposure distance was shortened to5 cm and the exposure time was prolonged to 40 minutes. The thickness ofthe film thus obtained was 4.7-4.8 μm. It has, accordingly, become about2.75 μm thicker during the UV irradiation.

The preceding examples can be repeated with similar success bysubstituting the generically or specifically described reactant and/oroperating conditions of this invention for those used in the precedingexamples.

From the foregoing description, one skilled in the art can easilyascertain the essential characteristics of this invention, and withoutdeparting from the spirit and scope thereof, can make various changesand modifications of the invention to adapt it to various usages andconditions.

What is claimed is:
 1. In a radiation-curable two-layer systemcomprising:a substrate having an outer surface; a first layer containinga polymerization photoinitiator, said first layer being applied on topof said outer surface of said substrate; and a second layer containingphotopolymerizable compounds superposed on said first layer, theimprovement wherein said photoinitiator is attached to said outersurface of said substrate by a covalent bond and wherein saidphotoinitiator is of the formula

    Ar-CO-R

wherein Ar is ##STR3## R is Ar or CR² R³ R⁴, R¹ is H, halogen, C₁ -C₁₂-alkyl, C₁ -C₁₂ -alkoxy, C₁ -C₁₂ -alkylthio or A, R² and R³independently of one another are each H, C₁ -C₆ -alkyl or phenyl, or oneof R² and R³ radicals is C₁ -C₆ -alkylene-O-A, or R² and R³ together areC₂ -C₆ -alkylene or C₁ -C₆ -oxaalkylene, R⁴ is OR⁵, N(R⁵)₂, SR⁵ or A, R⁵is H, C₁ -C₆ -alkyl or C₁ -C₆ -alkanoyl, A is --W-Si-R_(a) ⁶ R_(b) ⁷R_(c) ⁸, R⁶, R⁷ and R⁸ are each independently H, F, Cl, Br, I, CN, NC,OCN, NCO, SCN, NCS, N₃, C₁ -C₁₅ -alkoxy or C₁ -C₁₅ -alkanoyl, W is adirect bond or is C₁ -C₂₀ -alkylene or is C₁ -C₂₀ -alkylene wherein oneor two nonadjacent CH₂ groups is replaced by O, N or S, and a, b and care 0, 1, 2 or 3, subject to the provisos that: (1) a+b+c=3 and at leastone of the substituents R⁶, R⁷ and R⁸ is other than hydrogen and (2)said initiator contains at least one radical of the formula A,whereinsaid covalent bond is created between a reactive group on said outersurface of said substrate and a silane group A.
 2. A radiation-curabletwo-layer system according to claim 1, wherein said photopolymerizablecompounds comprise monoethylenically or polyethylenically unsaturatedmonomer.
 3. A radiation-curable two-layer system according to claim 1,wherein said substrate is made of paper.
 4. A radiation-curabletwo-layer systems according to claim 1, wherein said photopolymerizablecompounds comprise monoethylenically or polyethylenically unsaturatedoligomers.
 5. A radiation-curable two-layer system according to claim 1,wherein said photopolymerizable compounds comprise monoethylenically orpolyethylenically unsaturated prepolymers.
 6. A radiation-curabletwo-layer system according to claim 1, wherein said substrate is made ofcardboard.
 7. A radiation-curable two-layer system according to claim 1,wherein said substrate is made of wood.
 8. A radiation-curable two-layersystem according to claim 1, wherein said substrate is made of glass. 9.A radiation-curable two-layer system according to claim 1, wherein saidsubstrate is made of polymeric material.
 10. In a radiation-curabletwo-layer system comprising:a substrate having a surface; a first layercontaining a polymerization photoinitiator, said first layer beingapplied on said surface of said substrate; and a second layer containingphotopolymerizable compounds superposed on said first layer, theimprovement wherein said photoinitiator is attached to said surface ofsaid substrate by a covalent bond and wherein said photoinitiator is ofthe formula

    Ar-CO-R

wherein Ar is ##STR4## R is Ar or CR² R³ R⁴, R¹ is H, halogen, C₁ -C₁₂-alkyl, C₁ -C₁₂ -alkoxy, C₁ -C₁₂ -alkylthio or A, R² and R³independently of one another are each H, C₁ -C₆ -alkyl or phenyl, or oneof R² and R³ radicals is C₁ -C₆ -alkylene-O-A, or R² and R³ together areC₂ -C₆ -alkylene or C₁ -C₆ -oxaalkylene, R⁴ is OR⁵, N(R⁵)₂, SR⁵ or A, R⁵is H, C₁ -C₆ -alkyl or C₁ -C₆ -alkanoyl, A is --W-Si-R_(a) ⁶ R_(b) ⁷R_(c) ⁸, R⁶, R⁷ and R⁸ are each independently H, F, Cl, Br, I, CN, NC,OCN, NCO, SCN, NCS, N₃, C₁ -C₁₅ -alkoxy or C₁ -C₁₅ -alkanoyl, W is adirect bond or is C₁ -C₂₀ -alkylene or is C₁ -C₂₀ -alkylene wherein oneor two nonadjacent CH₂ groups is replaced by O, N or S, and a, b and care 0, 1, 2 or 3, subject to the provisos that: (1) a+b+c=3 and at leastone of the substituents R⁶, R⁷ and R⁸ is other than hydrogen, (2) saidinitiator contains at least one radical of the formula A, and (3) saidsubstrate is made of paper, cardboard, wood, or a polymericmaterial,wherein said covalent bond is created between a reactive groupon said outer surface of said substrate and a silane group A.